Make money from advertising on your website

Making money from your website using advertising - I am very new to this and to be honest i bearly know how to use a computer but i am keen to learn and i was just wondering in your opinion what you think the best type of internet site would be to create the most traffic and what is the best way to approach companies for banners ect. .do you make your own ads spots on the site ,i mean you decide where you want the ads and what ads formats you want to use and then just copy paste a codeline from the ads agengy ,or you have to work from the beginning with an ads agency to create these ads slots ? You may not have intended to make money from your site but now that you have an audience you realise that it's possible, or perhaps you have to start thinking about generating income because your costs to manage the site have increased and it's starting to hurt. . that i feel change the way of a certain market like youtube or myspace… how do you feel i should go about it… with advertising to bring in a income like those specifics sites… just to ask someone who already went through the ups and downs of the market.